2 Main Components of Peripheral Nervous System in Fishes

The following points highlight the two main components of peripheral nervous system in fishes. The components are: 1. Cranial Nerves 2. Spinal Nerves.

Peripheral Nervous System: Component # 1. Cranial Nerves:

There are ten pairs of cranial nerves found in the fishes (Figs. 12.12, 12.13 and 12.14).

These are as follow:

I. Olfactory Nerve:

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It originates from the olfactory bulb particularly from lamina terminalis. It innervates snout and is a special sensory which carries smell impulses to the brain.

II. Optic Nerve:

It takes origin from ventral side of optic tectum of midbrain. It innervates retina of eye. It is also a special sensory nerve and carries visual impulses.

III. Oculomotor:

It arises from optic tectum of midbrain (anterior brain stem under optic tectum), passes through the optic foramen and enters the orbit to supply superior, inferior, anterior, rectus and inferior oblique muscles of eyeball. It is somatic motor, innervates four of the six striated eye muscles and muscles inside eye.

IV. Trochlear:

It originates from dorsolateral side of brain stem under cerebellum. It innervates one of the six striated muscle of the eye, i.e., superior oblique muscle of eyeball.

V. Trigeminal:

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It arises from the lateral sides of the medulla oblongata. It is mixed somatic sensory and motor in function. It divides into deep ophthalmic, maxillary and mandibular branches. It innervates anterior part of head, upper and lower jaw regions.

VI. Abduscens:

It originates from the ventral sides of the medulla oblongata and runs just behind the trigeminal. It innervates sixth striated muscle, i.e., posterior rectus muscle, which moves the eyeball.

VII. Facial:

It arises from the sides of medulla oblongata. It is truly mixed nerve. It divides into three branches, supra-orbital, infra-orbital and the hyomandibular.

The facial VII and acoustic VIII arise from the medulla so intimately that they are often considered as acoustic facialis nerve.

IX. Glossopharyngeal:

It originates from the ventro-lateral side of medulla oblongata, just behind the VIII nerve. It is often fused with vagus X. It is a mixed nerve. It innervates ventral pharyngeal mucosa and muscles of first gill slit.

X. Vagus:

It takes origin from medulla just after the glossopharyngeal. It is mixed nerve.

Peripheral Nervous System: Component # 2. Spinal Nerves:

Each Spinal Nerve contains two roots:

The posterior sensory root with its ganglion and anterior motor root. The posterior root is composed mostly of sensory fibres from nerve cells in the spinal nerve ganglion and connects it with the C.N.S. However, the motor fibres passing through the anterior root have their cell bodies in the grey matter of the spinal cord.

With the help of stereotaxis apparatus, numerous nuclei in the forebrain are reported. These are as follows:

(i) Nucleus Anterioris Hypothalami (NAH):

It is present on the ventrolateral to the posterior part of the optic chiasma.

(ii) Nucleus Preglomerulosus Pars Lateralis (NPGL):

It lies lateral to the horizontal commissure and extends into posterior hypothalamus.

(iii) Nucleus Preglomerulus Pars Medialis (NPGm):

It appears medially and then ventromedially to the NPGL in the region posterior to the horizontal commissure.

(iv) Nucleus Preglomerulus Pars Medialis Commissuralis (NPGC):

It is placed medially at the posterior end of the nucleus glomerulosus.

(v) Nucleus Preopticus Periventricularis (NPP):

It is situated around the borders of the anterior extension of the preoptic recess of the third ventricle.

(vi) Nucleus Anterioris (Hypothalami) Periventricularis (NAPv):

This nucleus is present ventral to the nucleus preopticus in the region of the posterior end of the optic chaisma.